For the new study, researchers compared men aged 18 to 30 with men 55 and older who used exercise bikes for 30 minutes. During that time, blood samples were collected six different times, to assess cell function and antioxidant response.

Antioxidants are believed to protect healthy cells from damage caused by unstable molecules called "free radicals," according to the U.S. National Institutes of Health.

"Through this study, we were able to determine that an individual's antioxidant response to exercise becomes suppressed with age," researcher Tinna Traustadottir, an associate professor of biology at Northern Arizona University, said in a university news release.

"Exercise is effective and critical for people of all ages, but this study shows that older adults do not achieve the same beneficial cellular responses as younger adults from a single bout of moderate exercise," she said.

"By better understanding the molecular signals promoting beneficial effects of exercise, definitive recommendations could be made for improving the body's reaction to oxidative stress, which could lower the risk for many chronic diseases," the researchers said.

Oxidative stress involves the overproduction of oxygen byproducts that are toxic to cells.

Still, health experts advise seniors to make exercise a key part of their lives. According to the U.S. National Institutes of Health, studies have shown that exercise "provides many health benefits and that older adults can gain a lot by staying physically active.

"Even moderate exercise and physical activity can improve the health of people who are frail or who have diseases that accompany aging," according to the NIH.

The study was published in the July issue of the journal Free Radical Biology and Medicine.